At what point does copying code become copyright theft?

I understand this for normal language, but being a total novice at computer coding etc, I don't over here.

Could someone tell me if you go into a webpage, copy all its content, change the engligh language to make it talk about another topic, change maybe some colour or other, and set it up as your brand new site, can the other page send you to court? What about if you change a bit of code? And how do you go through your own code to see which is proprietary? Do you know any precedents that we could mull over?

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